The present invention generally relates to semiconductor devices and more particularly to a high speed bipolar transistor that uses a heterojunction of compound semiconductor materials.
Compound semiconductor materials such as GaAs or AlGaAs have a characteristic band structure that provides a large electron mobility. Thus, intensive efforts are being made on the research of semiconductor devices that use such compound semiconductor materials, in prospect of realizing high speed semiconductor devices. As the compound semiconductor materials generally form a mixed crystal over a wide compositional range, it is expected that such compound semiconductor devices provide the possibility of constructing various device structures tailored according to the specific needs. In this respect, compound semiconductor devices have distinct advantage over conventional semiconductor devices that are formed of single component semiconductor material such as Si or Ge. Thus, there are proposals to construct logic circuits or memory cells, conventionally formed of a large number of transistors, by means of single or small number of compound semiconductor devices.
FIG. 1 shows the construction of a conventional HET (hot electron transistor), which is a typical example of the compound semiconductor devices.
Referring to FIG. 1, the HET is constructed on a semi-insulating GaAs substrate 11 and includes a collector layer 12 of n-type GaAs formed on the substrate 11, a collector barrier layer 13 of undoped AlGaAs formed on the collector layer 12, and a base layer 14 of n-type GaAs formed on the collector barrier layer 13. On the base layer 14, an emitter barrier layer 15 of undoped AlGaAs is formed, and an emitter layer 16 of n-type GaAs is formed further on the emitter barrier layer 15. A part of the collector layer 12 is exposed and a collector electrode 12a is formed in correspondence to such an exposed part. Similarly, a part of the base layer 14 is exposed and a base electrode 14a is formed in correspondence to such an exposed part. Furthermore, an emitter electrode 16a is formed on the emitter layer 16.
FIG. 2 shows the band structure of the HET of FIG. 1 for the state in which a bias voltage is applied between the emitter electrode 16a and the collector electrode 12a.
Referring to FIG. 2, the electrons on the conduction band Ec of the emitter layer 16 cause a transit through the emitter barrier layer 15 by tunneling and are injected to the base layer 14 in the form of hot electrons. The hot electrons thus injected have energies much higher than the energy level of the band edge of the base layer 14, and are injected to the collector layer 12 by overriding the potential barrier formed in the conduction band of the collector barrier layer 13 with a barrier height .DELTA.Ec. On the other hand, the normal, cold electrons on the conduction band Ec of the base layer 14 are prevented from flowing to the collector layer 12 by the potential barrier .DELTA.Ec of the collector barrier layer 13. In such a structure, one can control the probability of tunneling of the electrons to pass through the emitter barrier layer 15 by the base voltage applied to the base electrode 14a. As a result of the control of the tunneling probability thus achieved, the collector current is controlled as a function of the base voltage.
In order to secure a large current gain in such conventional HETs, it is necessary to reduce the thickness of the base layer 14 as much as possible, such that the hot electrons do not lose energy in the base layer 14 as a result of scattering. When the hot electrons lose energy in the base layer 14, the chance that the electrons and holes cause recombination in the base layer 14 increases, while such a recombination of the carriers invites unwanted increase of the base current. In relation to the scattering of the carriers in the base layer 14, there arises another problem, in such a HBT, in that one cannot increase the concentration level of the impurities in the base layer 14 as desired, in view of the requirement for minimizing the impurity scattering of the carriers in the base layer 14. Thus, it will be easily understood that the fabrication of such a HET having the base layer 14 with extremely small thickness and low impurity concentration level, is extremely difficult. This difficulty becomes particularly conspicuous when a large number of HETs are assembled to form an integrated circuit.
Further, such a conventional HET operates satisfactorily only in the extremely low temperature environment where the thermal excitation of the electrons in the base layer 14 is small. When the environmental temperature increases, the electrons thermally excited in the base layer 14 tend to override the collector barrier .DELTA.Ec and flows into the collector layer 12.
In order to avoid such uncontrolled injection of the carriers, one may motivated to increase the barrier height .DELTA.Ec. However, use of such a large barrier height .DELTA.Ec necessitates a large emitter-collector voltage for the operation of the device. When a large emitter-collector voltage is used, the electrons tend to cause inter-band transition and the control of the transistor operation by way of the base voltage becomes impossible. From the foregoing reasons, it is believed that the practical operational temperature of HETs is limited below 77 K.
FIG. 3 shows the construction of another HET proposed by the inventors of the present invention. In FIG. 3, those parts corresponding to the structure of FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 3, the illustrated HET includes a first emitter layer 16.sub.1 of n-type GaAs and a second emitter layer 16.sub.2 also of n-type GaAs, wherein both of the first and second emitter layers 16.sub.1 and 16.sub.2 are formed on the common emitter barrier layer 15 with a mutual separation from each other, while it will be noted that the structure of FIG. 3 lacks the base electrode 14a. Further, first and second emitter electrodes 16a.sub.1 and 16a.sub.2 are formed respectively on the emitter layers 16.sub.1 and 16.sub.2.
In the transistor that lacks the base electrode as such, the electrons captured by the base layer upon injection of the electrons via one of the emitters, are removed from the base layer via the other of the emitters. Hereinafter, the operation of the HET of FIG. 3 will be described with reference to the band structure of FIG. 4.
Referring to FIG. 4, the majority of the hot electrons injected to the base layer 14 from the emitter layer 161 via the emitter barrier layer 15, reach the collector layer 12 after overriding the collector barrier layer 13 similarly to the case of FIG. 2. On the other hand, a part of the hot electrons may be scattered in the base layer 14 and captured by the base layer 14 as a result of recombination with the holes existing in the base layer 14. In the device of FIG. 3, the electrons thus accumulated in the base layer 14 are then removed therefrom via the emitter barrier layer 15 by applying a positive voltage to the second emitter electrode 16.sub.2 (with respect to the electrode 16.sub.1). Thereby, a current corresponding to the base current of the HET of FIG. 1 flows through the emitter 16.sub.2 as well as through the emitter electrode 16a.sub.2, and the transistor operates similarly to the ordinary HET. As will be easily noted from the structure of FIG. 3, such a HET of multiple emitter construction lacks the base electrode provided directly on the thin base layer and is easily fabricated, particularly in the form of integrated circuit.
Further, it should be noted that the single HET of FIG. 3 operates also as a logic circuit, which has conventionally been constructed from a number of devices, by changing the combination of the input voltages applied to the emitter electrodes 16a.sub.1 and 16a.sub.2 as will be described below.
FIG. 5 shows the equivalent circuit diagram corresponding to the transistor of FIG. 3, wherein the transistor of FIG. 5 is different from the transistor of FIG. 3 in that there are four emitter electrodes instead of the aforementioned two emitter electrodes 16a.sub.1 and 16a.sub.2.
Referring to FIG. 5, logic signals A-D are applied to the four emitter electrodes respectively, wherein the HET, having a symmetrical structure with respect to the plurality of emitters and emitter electrodes, operates symmetrically with respect to the input signals A-D and forms an exclusive NOR (XNOR) circuit. In other words, the single HET of FIG. 3 constructs a logic circuit that has hitherto been constructed from a plurality of transistors.
While the HET of FIG. 3 has such an advantageous feature of constructing complex logic circuits from small number of devices, the HET does have a problem in that the operation of the device in a high temperature environment exceeding the temperature of 77 K., such as the room temperature, inevitably causes an uncontrolled injection of thermally excited electrons from the base layer 14 to the collector layer 12. It is believed that this problem of room temperature operation is inherent to the principle of HET that distinguishes the hot electrons and the cold electrons by means of the collector barrier. In order to construct a device that is operational in the room temperature environment, one has to rely upon a transistor operating based upon a different principle.
FIG. 6 shows the construction of a HBT (heterojunction bipolar transistor), which is similar to the construction of a HET.
Referring to FIG. 6, the HBT is constructed upon a substrate 21 of semi-insulating GaAs and includes a collector contact layer 22 of n-type GaAs formed on the substrate 21 and a collector layer 23 of n-type GaAs formed on the collector contact layer 22. Further, a base layer 24 of p-type GaAs is formed on the collector layer 23, and an emitter layer 25 of n-type material having a large bandgap such as AlGaAs is formed on the base layer 24. Further, an emitter contact layer 26 of n-type GaAs is formed on the emitter layer 25. Furthermore, there is provided a graded layer 25a on the part of the emitter layer 25 that contacts with the base layer 24, such that the concentration level of A1 increases with increasing distance from the base layer 24. The surface of the contact layer 22 is exposed partially, and a contact electrode 22a is provided in correspondence to such an exposed part. Similarly, the surface of the base layer 22 is exposed partially, and a base electrode 24a is provided in correspondence to the exposed part. Further, an emitter electrode 26a is formed on the emitter contact layer 26.
FIG. 7 shows the band structure for explaining the operation of the conventional HBT, wherein the conduction band and the valence band are designated respectively by Ec and Ev.
Referring to FIG. 7, the electrons in the emitter layer 25 are injected to the base layer 24 through the graded layer 25a at the emitter-base interface. The electrons thus injected transit through the base layer 24 by drifting and reach the collector layer 23. The HBT of such a construction is characterized by a small base current due to the large bandgap of the emitter layer 25 as compared with the base layer 24, and achieves a large current gain as a result. Further, because of the large electron mobility of GaAs that forms the base layer 24, the HBT operates at a high speed. Furthermore, it should be noted that the base layer 24 and the collector layer 23 of the HBT are separated electrically from each other by means of a depletion region associated with the p-n junction, not by the barrier layer as in the case of HET. Thus, the HBT operates stably even in the room temperature environment.
However, the HBT of FIG. 6, having the base electrode 24a provided on the extremely thin base layer 24, has a problem similar to the HET of FIG. 1 in that the fabrication of the device is difficult. Further, the p-n junction, formed between the base layer 24 and the emitter layer 25, is exposed on the base layer 24 in the HBT of FIG. 6, while such an exposure of the emitter-base junction invites instability of operation of the device due to the surface states formed in correspondence to such a base-emitter junction by contamination.
In such a situation, a person skilled in the art would be motivated to eliminate the base electrode and provide, instead of the base electrode, a plurality of emitters and corresponding emitter electrodes on the HBT similarly to the HET of FIG. 3. Hereinafter, the problems that are encountered in such a multiple emitter HBT construction will be examined in detail.
First, the structure of a HBT shown in FIG. 8 will be described, wherein those parts described previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 8, the illustrated HBT includes a first emitter contact layer 26.sub.1 of n-type GaAs formed on the emitter layer 25 and a second emitter contact layer 26.sub.1 also of n-type GaAs provided on the same emitter layer 25 with a separation from the foregoing emitter contact layer 26.sub.1, wherein the emitter contact layer 26.sub.1 carries thereon a first emitter electrode 26a.sub.1 while the emitter contact layer 26.sub.2 carries thereon a second emitter electrode 26a.sub.2. When a typical HBT structure, shown in FIG. 6, is used for the device of FIG. 8, the collector layer 23 is formed of an n-type GaAs layer having a thickness of about 300 nm and an impurity concentration level of 1.times.10.sup.17 cm.sup.-3 while the base layer 24 is formed of a p-type GaAs layer having a thickness of about 100 nm and an impurity concentration level of 5.times.10.sup.18 cm.sup.-3. Further, the emitter layer 25 may be formed of an n-type AlGaAs layer having a thickness of about 200 nm and an impurity concentration level of 3.times.10.sup.17 cm.sup.-3. It should be noted that a part of the emitter layer 25 that contacts with the base layer 24 is formed of a graded layer 25a having a thickness of about 20 nm and a composition represented by Al.sub.x Ga.sub.1-x As, wherein the compositional parameter x changes generally linearly in the graded layer 25a from the upper major surface of the base layer to the upper boundary to the emitter layer 25. It should be noted that the compositional parameter x reaches a value of about 0.30 at the upper boundary of the graded layer 0.30. By providing such a graded layer between the base layer 24 and the emitter layer 25, it is possible to eliminate the discontinuous change or kink of the conduction band and the valence band at the heterojunction interface.
FIG. 9 shows a band diagram for explaining the operation of the HBT of FIG. 8. In FIG. 9, it should be noted that the conduction band is designated as Ec while the valence band is designated as Ev.
Referring to FIG. 9, the electrons are injected from one of the emitter electrodes and a corresponding emitter contact layer, such as the emitter electrode 26a.sub.1 and the emitter contact layer 26.sub.1, to the emitter layer 25 that includes the graded layer 25a therein, wherein the electrons thus injected to the emitter layer 25 are further injected to the base layer 24 and reach the collector layer 23 after crossing the base layer 24 under a suitable bias condition, similarly to ordinary bipolar transistors. Thereby, a part of the electrons injected to the base layer 24 cause a recombination with holes in the base layer 24, and as a result, there occurs a depletion of holes and corresponding excess of electrons in the base layer 24. In the conventional bipolar transistor shown in FIG. 6, such excess electrons are removed from the base layer 24 via the base electrode 24a in the form of the base current.
As the HBT of FIG. 8 does not use the base electrode 24a, such a removal of the electrons from the base electrode by way of the base electrode is not possible. On the other hand, it may be possible to remove the excess electrons from the base layer 24 by using the emitter electrode 26a.sub.2 and corresponding emitter contact layer 26.sub.2 as a base.
FIG. 9 shows the band diagram for explaining such an operation of the HBT. It should be noted that FIG. 9 shows the band structure taken along a line connecting the emitter contact layer 26.sub.1, the emitter layer 25, the graded layer 25a, the base layer 24, the graded layer 25a, the emitter layer 25 and the emitter contact layer 26.sub.2.
Referring to FIG. 9, there is formed a depletion layer at the junction interface between the n-type emitter layer 25 (the graded layer 25a included) and the p-type base layer 24 as is well known in the art, wherein the electrons are easily injected from an emitter region E.sub.1, corresponding to the emitter electrode 26a.sub.2 and the emitter contact layer 26.sub.2, to the base layer 24 through the depletion layer, in the forward biasing state of the emitter region E.sub.1. On the other hand, a region E.sub.2 of the emitter 25 corresponding to the emitter electrode 26a.sub.2 and the emitter contact layer 26.sub.2, is applied with a reverse biasing, and because of this, the depletion layer formed between the emitter region E.sub.2 and the base layer 24 has a substantial thickness. Thus, the transit of the electrons through such a depletion layer is difficult. In other words, it is difficult to remove the electrons from the base layer in the HBT having the structure of FIG. 8, even when one uses the emitter contact layer 26.sub.2 and the emitter electrode 26a.sub.2 in place of the base of ordinary HBT.